ArrayList add, get, set, and size

A plain array is born with a fixed length and can never change it. An ArrayList is an object that manages a hidden array behind the scenes, so when you add more elements than it currently holds, it quietly builds a bigger array and copies everything over. You never see that resizing happen; you just call add and the list handles it.

Each method has one job. add(x) appends x after the last element, growing the list by one; add(i, x) instead inserts x at index i and shifts everything from i onward one spot to the right. get(i) returns the value sitting at index i without changing anything. set(i, x) overwrites whatever is at index i with x and returns the old value, but it does not change how many elements exist. size() is a method call that returns the current element count, recalculated every time you ask.

This matters because indices still run from 0 to size()-1, exactly like array indices run from 0 to length-1, so all the off-by-one habits from arrays carry straight over. The difference is that size() is a live count you query with parentheses, not a stored number you read off a field. Once you're comfortable with these four methods, the next step is using them inside loops to traverse, search, and shift elements, which is exactly what array algorithms like linear search and insertion do, just written with method calls instead of bracket notation.